Apparatus for tuning an attenuating phase shift mask

ABSTRACT

The present invention provides a method and an apparatus for tuning the phase shifting of a phase shift mask having an attenuating phase shifting material providing at least 160° of phase shift. The method comprising the utilization of a phase measurement and selective etching operations which reduces the challenge of making an exacting 180° phase shifting mask. An attenuated phase shift mask structure is also disclosed in the present invention.

RELATED APPLICATIONS

This application Ser. No. 09/225,128, filed Jan. 4, 1999, U.S. Pat No.6,027,837, which is a continuation-in-part application of U.S.application Ser. No. 08/949,916, filed Oct. 14, 1997 abandoned.

TECHNICAL FIELD

The present invention relates to optical lithography and, in particular,to a method and an apparatus for tuning the phase shifting of a phaseshift mask having an attenuating phase shifting material providing atleast 160° of phase shifting. Specifically, the method of the presentinvention is capable of directly tuning a phase shifting material by thecontrolled removal of: (i) portions of a transparent or semi-transparentsubstrate on which said phase shifting material is applied to; (ii)portions of the phase shifting material; or (iii) portions of both thetransparent or semi-transparent substrate and the phase shiftingmaterial. The controlled removal allows for tuning a phase shift mask toabout 180° directly during the mask's fabrication. The present inventionalso relates to an attenuating phase shift mask structure which has aphase measurement of between about 170° to about 190°.

PRIOR ART

Phase shifting technology is one of the useful methods available toenhance the performance of optical lithography. An attenuating phaseshifting mask normally consists of an absorbing phase shifting materialhaving from about 5 to about 20% transmittance and a transparent orsemi-transparent substrate. The absorptive shift patterns have beenformed with a two layered structure comprising a SiO₂ film to controlthe phase and a thin Cr film to control the transmittance.

A single layer shifter comprising an oxide or an oxynitride film ofMoSi, described by Yoshioka, et al. "Practical attenuated phase-shiftingmask with a single-layer absorptive shifter of MoSiO and MoSiON for ULSIfabrication", IEEE, IEDM 93 pp. 653-656, is also known. This attenuatingphase shift mask is fabricated by steps comprising: sequentiallyapplying a MoSiO shifter, a Mo film and a resist to a glass substrate;developing the resist; etching the exposed regions of the glasssubstrate by reactive ion etching (RIE) in a gas mixture of CF₄ and O₂ ;removing the resist by utilizing an O₂ plasma; and wet etching the Mofilm layer. The single layer attenuating phase shift mask produced bythe method described in Yoshioka, et al. has a 180° phase shift and atransparency of between about 5 and 20%. The desired phase shifting isachieved in the disclosure of Yoshioka, et al. by their selection of asubstrate material that has a phase shift of about 180°. Deviations fromthis 180° phase shift can occur during the fabrication process disclosedin Yoshioka, et al.

As is well known to those skilled in the art, an attenuating phase shiftmask can produce a high resolution image on a wafer when the attenuatingphase shift mask is fabricated to a tight 180° (relative to the maskusage wavelength) tolerance. Variations in the starting phase shift filmand the means in which the same is fabricated may cause the mask to havean error from the desired 180° phase shift.

As is disclosed in Yoshioka, et al., the prior art requires multipleprocessing steps in forming their attenuating phase shift masks. Thesemultiple processing steps may introduce a significant phase shift fromthe desired 180°. That is, the prior art processes do not account formean phase errors which arise during the fabrication of the attenuatingphase shift mask. There is thus a need to develop a new and improvedmethod of fabricating an attenuating phase shift mask that compensatesfor the mean phase error in the phase shifting material. Specifically,there is a need for developing a method for tuning the mask to 180°during the fabrication process which eliminates all of the drawbacksmentioned hereinabove with prior art processes.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a method for tuning anattenuating phase shift mask to an exact phase.

Another object of the present invention is to provide a method whichcompensates for phase errors in the attenuating phase shifting materialthat may occur during the fabrication of the attenuating phase mask.

A further object of the present invention is to provide a method offabricating an attenuating phase mask that is tuned to about180° with ahigh degree of accuracy.

Yet a further object of the present invention is to provide an apparatusfor fabricating a phase shift mask that is tunable to about 180°.

In one embodiment of the present invention, the above as well as otherobjects are achieved by utilizing a method which includes the controlledremoval of portions of the transparent or semi-transparent substrate onwhich the phase shifting material is applied so as to provide 20° orless of phase shift in the substrate. Specifically, in this aspect, thepresent invention relates to a method for tuning the phase shifting of aphase shift mask having an attenuating phase shifting material providingat least 160° of phase shifting, said method comprises the steps of:

(a) forming line segment images in a phase shifting material, said phaseshifting material being located on an area of a transparent orsemi-transparent substrate;

(b) measuring the actual phase shift provided by the imaged phaseshifting material; and

(c) etching the substrate in areas not covered by said imaged phaseshifting material by an amount sufficient to provide about180° of phaseshift.

In accordance with the above embodiment of present invention, step (a)is carried out by spinning a photoresist on a layer of a phase shiftingmaterial which has been previously applied to a transparent orsemi-transparent substrate; writing lines, i.e., patterning, onpredetermined areas of the photoresist; developing the photoresist; andetching the developed photoresist and areas of the phase shiftingmaterial not containing the desired pattern. In step (b) of the presentinvention, a laser beam and a phase interferometer are utilized todetermine the actual phase shift caused by the phase shifting material.Step (c) of the present invention includes a wet etch, a dry etch orcombination of the two and it selectively removes 20° or less of phaseshift from the transparent or semi-transparent substrate so as to tunethe imaged phase shifting material to about 180°.

In another embodiment of the present invention, the phase shift of aphase shift mask having an attenuating phase shifting material providingat least 160° of phase shifting is tuned by a method which comprisesconducting steps (a)-(b) above and then (c') etching a portion of theimaged phase shifting material by an amount sufficient to provide about180° of phase shift to the phase shifting material. This embodiment ofthe present invention is employed when the attenuating phase shiftingmaterial has a measured phase that is above a desired predeterminedvalue.

In yet another embodiment of the present invention, a combination ofcontrolled etching of the substrate and the phase shifting material maybe used to tune the phase shifting material. In accordance with thisaspect of the present invention, the method comprises the steps of:

(a) forming line segment images in a phase shifting material, said phaseshifting material being located on an area of a transparent orsemi-transparent substrate;

(b) measuring the actual phase shift provided by the imaged phaseshifting material;

(c) etching the substrate in areas not covered by said imaged phaseshifting material by an amount sufficient to provide about 180° of phaseshift, or

(c') etching a portion of the imaged phase shifting material by anamount sufficient to provide about 180° of phase shift; and

(d) repeating steps (b) and (c) or (c') any number of times until a 180°phase shift is obtained.

Another aspect of the present invention relates to an apparatus fortuning the phase shifting of a phase shift mask having an attenuatingphase shifting material providing at least 160° of phase shifting.Specifically, the apparatus of the present invention comprises:

(a) means for measuring the actual phase shift provided by a line imagedmask, said line imaged mask comprising a phase shifting materialproviding at least 160° of phase shifting located on a transparent orsemi-transparent substrate;

(b) means for etching said transparent or semi-transparent substrate toa depth to provide about 20° or less of phase shift in said substrate;and

(c) means for providing a feedback when said etching is completed.

In alternative embodiments, means (b) may be replaced by means (b')which includes a means for etching the phase shifting material to adepth to provide about 0.01° to about 20° of phase shift in said phaseshifting material, or means (b) and (b') may be used in conjunction witheach other.

Another aspect of the present invention relates to an attenuating phasemask structure comprising a transparent or semi-transparent substratehaving an attenuating phase shift material on a portion of saidsubstrate, wherein said substrate, said phase shift material or bothcontains areas etched to provide about 0.01° to about 20° phase shift insaid substrate or in said phase shift material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-(d) are cross-sectional views of an attenuating phase shiftmask prepared in accordance with one embodiment of the presentinvention: (a) mask structure prior to conducting step (a) of thepresent invention; (b) mask structure after conducting step (a); (c)mask structure after wet etching; and (d) mask structure after dryetching.

FIG. 2 illustrates means for determining the transmittance differencebetween the imaged mask and the transparent or semi-transparentsubstrate.

FIG. 3 illustrates one embodiment of the apparatus of the presentinvention.

FIGS. 4(a)-(c) are cross-sectional views of an attenuating phase shiftmask prepared in accordance with an alternative embodiment of thepresent invention: (a) mask structure prior to conducting step (a) ofthe present invention; (b) mask structure after conducting step (a); and(c) mask structure after dry etching.

DETAILED DESCRIPTION OF THE INVENTION

The present invention which provides a method and an apparatus fortuning the phase shifting of a phase shift mask as well as a phase maskstructure will now be described in greater detail with reference to theaccompanying drawings wherein like reference numerals are used for likeand corresponding elements of the drawings.

As stated above, one aspect of the present invention relates to a methodfor tunning the phase shifting of a phase shift mask having anattenuating phase shifting material providing at least 160° of phaseshifting therein. In accordance with step (a) of the present invention,line segment images are formed in a phase shifting material which ispositioned on a transparent or semi-transparent substrate. Specifically,as shown in FIG. 1(a), the line segment image is formed as follows:

First a phase shifting material 12 is applied to one surface oftransparent or semi-transparent substrate 10 utilizing conventionaltechniques well known to those skilled in the art. For instance, thephase shifting material may be deposited onto the substrate by utilizingconventional chemical vapor deposition (CVD) or direct current (DC)sputtering.

Suitable materials which may comprise transparent or semi-transparentsubstrate 10 include, but are not limited to: quartz, glass, calciumfluoride, diamond, diamond-like carbon and the like. A highly preferredmaterial employed in the present invention as substrate 10 is quartz.

Transparent or semi-transparent substrate 10 employed in the presentinvention may be used as is, or it can be cleaned prior to use utilizingtechniques well known to those skilled in the art that are effective forremoving any contaminants from the substrate.

It is noted that in the prior art, it is critical that the substratematerial of the phase shift mask have a phase shift as close to 180° aspossible. This is so, since any significant deviations from 180° wouldresult in a significant phase error that could not be compensated forduring the manufacturing of the attenuating phase mask. In the presentinvention, it is not critical to use a substrate material whichinitially has a phase shift of 180°. Typically, the substrate materialsemployed in the present invention have an initial phase shift of fromabout 170° to about 178°.

Any attenuating phase shifting material well known to those skilled inthe art that is semi-transparent and has at least a 160° of phase shiftcan be used as layer 12. Examples of phase shifting material whichsatisfy the above criteria and thus can be used in the present inventioninclude, but are not limited to: MoSi, carbon, chrome oxide, chromenitride, silicon nitride, chrome fluoride, chrome oxide fluoride and.the like. A highly preferred phase shifting material that is employed inthe present invention is MoSi.

After applying the phase shifting material 12 to substrate 10, aphotoresist 14 is spun onto layer 12 utilizing conventional techniqueswell known to those skilled in the art. Any photoresist which is capableof being written on, i.e. patterned, and is sensitive to e-beam or laserlight exposure may be employed in the present invention. This includespositive photoresists as well as negative photoresists. Illustrativeexamples of suitable photoresists that may be employed in the instantinvention include, but are not limited to: poly(butene-1-sulphone) (PBS)and 895i, which is a positive novolak resist.

Next, a desired pattern is provided in the photoresist utilizingconventional e-beam or laser write techniques well known to thoseskilled in the art. For example, one way of writing a desired patterninto photoresist 14 is by e-beam writing a reference data design using araster-scan. This technique transfers the designed pattern from the datadesign into photoresist 14.

After providing the desired pattern to photoresist 14, the pattern isdeveloped utilizing conventional methods which include, but are notlimited to: Using sodium hydroxide, tetramethylammonium hydroxide(TMAH), methylisobutylketone (MIAK) with isopropyl alcohol or mixturesthereof. This creates a resist structure which contains a patternthereon. Next, the mask is etched utilizing standard etching techniqueswell known to those skilled in the art to provide the structure shown inFIG. 1(b) which comprises substrate 10 having an imaged phase shiftingmaterial, i.e. the mask, 12' thereon. It is noted that this etching stepof the present invention is carried out using a dry plasma etch that ishighly selective for the phase shifting material. Almost no etching ofsubstrate 10 occurs in this step of the present invention. Suitable dryplasma etchants that are highly selective for the phase shiftingmaterial include, but are not limited to: CF₄, CHF₃, O₂, SF₆ andcombinations thereof.

In accordance with step (b) of the present invention, the actual phaseshift provided by imaged phase shifting material 12' is then measured byutilizing the means shown in FIG. 2. Specifically, in FIG. 2, there isshown a laser source 20, a beam splitter 22, the structure of FIG. 1(b)and a phase interferometer 24. The measurement is made by firstsplitting laser beam 26 from laser source 20 into two beams, 26' and26", respectively, by passing beam 26 through beam splitter 22. One ofthe split beams, e.g., 26", is focused on a portion of substrate 10 andthe other, e.g., 26', is focused on imaged phase shifting material 12'.The two beams exiting the structure are then passed through phaseinterferometer 24 wherein the difference between the two beams, which isrepresentative of the phase shift of the imaged phase shifting material,is determined.

Using the means shown in FIG. 2, the actual phase shift of the imagedphase shifting material can be determined and used to tune the imagedmask to about 180° with high accuracy by utilizing, in this embodiment,a controlled etching regime to remove portions of substrate 10 notcontaining imaged phase shifting material 12'. The controlled etching isperformed by utilizing a dry etch, a wet etch or a combination thereof.It is emphasized that the controlled etching step of the presentinvention, step (c), is highly selective for etching transparent orsemi-transparent substrate 10. In accordance with the present invention,the conditions employed in this step of the present invention aresufficient to remove enough substrate so as to provide a phase shift offrom about 0.01° to about 20° in said substrate. The amount of materialto be removed is determined by the deviation of imaged phase shiftingmaterial 12' from 180°. For example, if the imaged mask has a phaseshift of 175°, the mask can be tuned by selectively removing portions ofsubstrate 10 so as to provide a 5° phase shift in the substrate. A 180°tuned mask is, thus, formed.

Suitable dry etching techniques that can be employed in the presentinvention to selectively remove portions of substrate 10 include:reactive ion etching (RIE), chemical dry etching, ion beam etching,plasma etching and the like. The gases which may be employed in thesedry etching techniques are those that have a high affinity for thetransparent or semi-transparent substrate. Examples of suitable gasesthat can be employed in the present invention are CF₄, SF₆, NF₃, CHF₃and combinations thereof. The gases may also be used in conjunction withoxygen or other chemicals such as Ar, N₂ and He.

When a wet etch is employed to selectively etch transparent orsemi-transparent substrate 10 containing imaged phase shifting material12', the chemical etchants employed include: HF, NaOH, buffered HF andthe like. Mixtures of these chemical etchants alone or with water arealso contemplated herein. The chemical etchants may also be buffered toa desired pH using well known buffering agents. A highly preferredchemical etchant employed in the present invention is buffered HF.

It is noted that the above described selective etching step of thepresent invention is conducted to provide a mask having a phase shift ofabout 180°. Typically, this controlled etching step removes from about10 to about 300 angstroms of the substrate material. The final structureformed from the present invention is shown in FIGS. 1(c) and (d).Specifically, FIG. 1(c) shows an attenuating phase mask of the presentinvention wherein the substrate is etched by utilizing a chemicaletchant. It is noted that undercuts 18 are present in this structuresince a chemical etchant is used in step (c) In FIG. 1(d), theattenuating mask was tuned by etching the substrate with a dry etchprocess--no undercut is present.

Reference is now made to FIGS. 4(a)-(c) which shown an alternativeembodiment of the method of the present invention. In the alternativeembodiment, the phase shift of image phase shifting material 12' istuned by selectively removing, i.e. etching, the phase shift materialitself. This embodiment of the present invention is employed when themeasured phase shift of imaged phase shift mask 12' is greater than aselected target value. It is noted that in this embodiment of thepresent invention steps (a) and (b) mentioned hereinabove in regard toFIG. 1 are first performed and then the phase shifting material isselectively etched instead of the substrate.

The imaged phase shifting material is selectively etched in the presentinvention by utilizing an etch process which is highly selective for thephase shifting material. Dry etching, chemical etching or a combinationthereof may be employed to selectively etch the phase shifting material.In FIG. 4(c), dry etching was used to remove portions of imaged phaseshifting material 12'. In a typical process, from about 10 to about 100Angstroms of imaged phase shifting material is removed.

Suitable dry etching processes that can be employed in etching theimaged phase shifting material include those mentioned hereinabove. Theselective etch is accomplished by changing the pressure of the etchingtechnique. While low pressures of from about 5 to about 50 mTorr areemployed in selectively dry etching the substrate, higher pressures offrom about 50 to about 300 mTorr are employed in selectively dry etchingthe imaged phase shifting material.

In accordance with one aspect of the present invention, the selectivedry etch removes from about 10 to about 300 Å of said transparent orsemi-transparent substrate. In another aspect, the selective dry etchingremoves from about 10 to about 100 Å of said imaged phase shiftingmaterial.

When a wet etch is employed to etch imaged phase shifting material 12',the chemical etchants employed include: HF and HNO₃. Mixtures of thesechemical etchants alone or with water are also contemplated herein. Thechemical etchants may also be buffered to a desired pH using well knownbuffering agents. A highly preferred chemical etchant employed in thepresent invention to etch the imaged phase shifting material is amixture of HF and HN0₃.

In addition to employing each of the above selective etch processesseparately, it is also contemplated by the present invention to employboth together. In accordance with this embodiment of the presentinvention, steps (a)-(b) are first carried out then either the substrateor the phase shifting material is selectively etched. The phase shift ofthe mask is then measured again using step (b) and then, if necessary,etching of the substrate or the phase shifting material may be carriedout. It is emphasized that the substrate is etched if the measured phaseshift is below a predetermined value, whereas the imaged phase shiftingmaterial is etched if the phase shift is above a predetermined value.

In accordance with the second aspect of the present invention, anapparatus for tuning the phase shifting of a phase mask is provided.Specifically, the apparatus of the present invention comprises:

(a) means for measuring the actual phase shift provided by a line imagedmask, said line imaged mask comprising a phase shifting materialproviding at least 160° of phase shifting located on a transparent orsemi-transparent substrate;

(b) means for etching either said transparent or semi-transparentsubstrate, said phase shifting material or both to a depth to provide aphase shift of about 20° or less;

(c) means for providing a feedback when said etching is completed; and

(d) optionally, means for stopping the etching.

Reference is made to FIGS. 2 and 3, which illustrate the apparatus ofthe present invention. Specifically, FIG. 2 which was discussed ingreater detail hereinabove represents the measurement means of theapparatus of the present invention. The measurement means shown in FIG.2 is employed in FIG. 3, which illustrates all of the means of theapparatus of the present invention. In addition to including lasersource 20, beam splitter 22, phase interferometer 24 and imaged phaseshift material 12' and substrate 10, the apparatus of the presentinvention also includes a housing means 30 wherein either the dry or wetetch of substrate 10 occurs, a feedback device 32 such as a computer oralarm which indicates when the phase of the structure is within180°, anda robot arm 34 for removing the tuned mask from the etchant medium. Whena wet etchant is employed the housing means includes a suitable chemicaletchant that is highly selective for the transparent or semi-transparentsubstrate. On the other hand, when a dry etch is performed the housingmeans includes a source region for bombarding the imaged mask with asuitable etchant gas.

The above description illustrates the method and apparatus of thepresent invention, as well as various phase shift mask structures thatcan be formed from the same. It is noted that the method of the presentinvention provides a means for directly tuning an attenuating phaseshift mask to about 180° with a high degree of accuracy.

While the invention has been particularly shown and described withrespect to the preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details made by made without departing from the scope and spirit ofthe present invention. It is therefore intended that the invention benot limited to the exact forms described and illustrated, but should beconstructed to cover all modifications that may fall within the scope ofthe appended claims. For example, this principle applies to masksutilizing technology with wavelength below light, such as X-rays.

Having thus described our invention, what we claim as new, and desire tosecure by the Letters Patent is:
 1. An apparatus for tuning the phaseshifting of an attenuating phase shift mask having a phase shiftingmaterial providing at least 160° of phase shifting, said apparatuscomprising:(a) means for measuring the actual phase shift provided by aline imaged mask, said line imaged mask comprising a phase shiftingmaterial providing at least 160° of phase shifting located on atransparent or semi-transparent substrate, said measuring meansincluding a laser source, a beam splitter position in front of saidlaser source and a phase interferometer position behind said beamsplitter, said line imaged mask being positioned between said beamsplitter and said phase interferometer whereby a laser beam from saidlaser source is split into a first beam and a second beam by said beamsplitter, said first beam is focused on a portion of said substrate andsaid second beam is focused on said phase shifting material, said firstand second beams exiting said substrate and said phase shifting materialbeing passed through said phase interferometer wherein any differencebetween the first and second beams, which is representative of the phaseshift of said phase shifting material, is determined; (b) means forselectively etching said transparent or semi-transparent substrate to asufficient depth so as to tune said phase shifting of said mask to about180°; and (c) means for providing a feedback when said tuning has beencompleted.
 2. The apparatus of claim 1 wherein means (b) is replacedwith (b'): means for selectively etching the imaged phase shiftingmaterial to a sufficient depth so as to tune said phase shifting of saidmask to about 180°.
 3. The apparatus of claim 1 further comprising (b')means for selectively etching the imaged phase shift material to asufficient depth so as to tune said phase shifting of said mask to about180°.
 4. The apparatus of claim 1 wherein said selective etching meansincludes a dry etch or a wet etch housing means.
 5. The apparatus ofclaim 4 wherein said dry etch housing means includes a source forbombarding the imaged mask with a gaseous etchant.
 6. The apparatus ofclaim 4 wherein said wet etch housing means includes a chemical etchant.7. The apparatus of claim 1 wherein said feedback means includes acomputer or an alarm device.
 8. The apparatus of claim 1 furthercomprising (d) a means for stopping said etching.
 9. The apparatus ofclaim 8 wherein said means for stopping said etching includes a robotarm.